High Temperature Gas Cooled Reactors (HTGRs) have attracted worldwide interest because of their high outlet temperatures, which allow them to be used for applications beyond electricity generation. Experimental and demonstration reactors of this type have operated in China, Great Britain, Germany, Japan, and the United States of America. During their operation research reactors such as the pebble bed type AVR (‘Arbeitsgemeinschaft Versuchsreaktor’) in Germany or the prismatic block type HTTR (High Temperature Test Reactor) have reached coolant temperatures of 990 °C (AVR) and 950 °C (HTTR). While the AVR stopped operation in the late 1980’s the HTTR demonstrated operation at such high temperatures (950 °C) without interruption (50 days non-stop) in 2010.

Whilst these high temperatures attained are very promising in terms of possible applications and enhanced economics, it may raise some concerns regarding the challenges to the coated particle fuel and structural materials. This applies mainly to the reactor unit but specific attention may also be required for some of the materials used in the power conversion unit. The fuel performance of coated particle fuels at high temperatures has been the focus in previous technical meetings (2012 and 2013) and therefore this meeting focussed on the high temperature structural materials. These include inter alia the graphite structures, carbon-fibre composites, and metallic materials (reactor pressure vessel, barrels, control rods and other in-core materials). The area of reactor application addresses HTGRs, experience from Gas Cooled Reactors (GCR) and other High Temperature Reactors (HTRs) such as those which use molten-salt-coolants.

The meeting concluded that graphite and metals for HTGRs are already available today for near term deployment at gas outlet temperatures up to 800oC. For higher temperatures work is in progress to have materials qualified in the foreseeable future. New graphite grades are being tested as part of the NGNP and Innograph programs and can also form the basis for the establishment of a new code in ASME. Similarly codes and standards are already available for reactor vessels metallic for near term applications (up to 800oC) and higher temperature materials such as Alloy 617 is being developed and added to the ASME code as part of the VHTR activities and will be available soon. A large amount of information is also available in the IAEA International Knowledge Base on Nuclear Graphite.